抄録
Subcritical crack growth is considered to be a source of the time-dependent fracture in rocks. Thus, it is very important to know the nature of subcritical crack growth in rocks. Although rocks are generally anisotropic, rocks have been assumed as isotropic materials in many studies. Therefore, anisotropic properties of subcritical crack growth in rocks are not known well. In this study, Double-Torsion test was carried out using three kinds of granite to investigate the anisotropic properties and the grain-size dependency of subcritical crack growth. Double-Torsion specimens were prepared from each granitic rock with considering the propagation direction and the opening direction of the crack.
It was shown experimentally that the subcritical crack growth behavior in granite was anisotropic. When the crack advanced parallel to RIFT plane, the stress level at the crack tip was the smallest in all granite used in this study. Additionally, the stress level at the same crack velocity was in good agreement when the opening direction of the crack was the same.
It was also indicated that subcritical crack growth in granite was dependent on the grain size. For coarser grained granite, the stress level causing crack growth tended to be lower than that of fine-grained granite.
It could be considered that anisotropy and grain-size dependency of subcritical crack growth in granite related to the preferred orientation and the density of pre-existing microcracks, because it was recognized that as the velocity of P-wave propagating in the opening direction of the crack was low, that is, as the crack density of granite was high, the stress level causing subcritical crack growth was low. It can be concluded that the subcritical crack growth behavior in granite is affected strongly by the orientation distribution of pre-existing microcracks.
